The use of a substantially C-shaped solder pad, with plated via holes for providing an electrical connection between various layers of a printed circuit board, wherein the C-shaped solder pad partially surrounds a non-plated through-hole which is resistant to solder occlusion upon wave soldering is known in the art and is disclosed in U.S. Pat. No. 4,851,614 ('614 patent), which is incorporated herein by reference. The use of a spoked solder pad is also known in the art.
Although these prior art solder pad designs tend to prevent the occlusion of solder in the printed circuit board throughholes, these designs also have some performance drawbacks which, up to this point, have not been addressed.
The C-shaped solder pad design of U.S. Pat. No. 4,851,614 is an asymmetric design with a preferred orientation. As stated in column 3, lines 24-26 of the '614 patent, the pad "is substantially C-shaped with the open portion oriented such that it is the first portion contacted by the solder when the board is wave soldered."
It has been discovered that the thickness of the solder adhering to the C-shaped solder pad varies if the orientation of the C-shaped solder pad design deviates from the preferred orientation, or from a predetermined orientation. In other words, the only way to ensure that variations in solder thickness will not occur is to make sure that every C-shaped solder pad surrounding every through-hole required to be non-occluding on every printed circuit board undergoing the wave soldering process is oriented with the opening in the "C" shape facing in the same direction. To ensure that every C-shaped solder pad is properly oriented adds time and expense to the design of the printed circuit board because of necessary coordination with manufacturing to predict the required orientation.
If all of the C-shaped solder pads are not properly oriented, then the variations in thickness that occur, while minor, can create problems in certain applications due to the precise tolerance requirements necessary when developing printed circuit boards for certain uses, such as in portable computers.
These variations in thickness raise other concerns as well. For example, when mounting the printed circuit board to a computer system chassis, a mounting bolt is inserted through the through-hole of the printed circuit board and is then tightened into a corresponding bolt hole on the chassis. As the mounting bolt is tightened, a lower surface of a mounting bolt head begins exerting pressure on the printed circuit board in the region of the through-hole, as well as on the solder pad located on the lower surface of the printed circuit board which comes into contact with the computer chassis. The thickness of the solder which comprises the solder pad is uneven around the through-hole and causes the uneven application of pressure around the through-hole by the tightening of the mounting bolt, and such uneven strain placed on the printed circuit board when the mounting bolt is tightened can result in the delamination of the printed circuit board in the region of the through-hole, as well as the separation of the solder pad or underlying copper from the surface of the printed circuit board. Also, these thickness variations contribute to mechanical dimensional tolerances, which may lead to the misalignment of circuit components on the printed circuit board to associated mechanical components on the computer chassis.
Even if there are no variations in thickness of the solder, which assumes proper orientation of the solder pads, the asymmetric design of the C-shaped solder pad inherently leads to the possibility of uneven pressures being applied to the printed circuit board around the through-hole by the tightening of the mounting bolt. Since solder attaches only to the unmasked exposed C-shaped portion of the copper plated ring, while the gap or opening which defines the "C" shape remains free of solder, there is inherently an uneven distribution of solder about the through-hole due to the asymmetry of the solder pad design. This uneven distribution of solder can create the same problems of delamination and separation as discussed above.
As mentioned previously, the use of the spoked solder pad is known in the art, although applicant does not know whether the spoked solder pad design was implemented as a solution to the problems arising from the C-shaped solder pad design of the prior art, as discussed above. Regardless, the spoked solder pad designs of the prior art also have drawbacks which, to this point, have not been addressed.
By implementing the spoked solder pad design, the problems of orientation are no longer a concern, since a symmetric design does not have a preferred orientation, and the direction of travel over a wave solder does not matter. In other words, a symmetric design is omni-directional.
However, the spoked solder pad design has a smaller contact area of solder between the printed circuit board and the computer chassis than the C-shaped solder pad design (as shown in comparison in FIG. 1a and FIG. 1b). The contact area of solder between the printed circuit board and the computer chassis via the solder pad serves the important function of providing an electrical connection between the logical ground of the circuit on the printed circuit board and the electrical ground of the computer chassis.
Due to the minimal contact area of the spoked solder pad design, certain drawbacks exist. In certain situations, if the contact area of solder on the solder pad between the printed circuit board and the chassis is insufficient to create a proper grounding connection, then the improper grounding which results can cause computer system errors.
Because of certain regulatory requirements regarding electro-magnetic interference (EMI) and radio frequency interference (RFI), proper grounding is imperative. For example, high frequency radio frequency (RF) energy must be adequately returned to the chassis electrical ground to prevent excessive radiation from the unit. This is in order to comply with specified emission levels (measured in decibels or dB) set by regulatory agencies. The total contact area between the exposed conductive area of the solder pad and the computer chassis is key to effectively maximizing the conduction of the RF energy to electrical ground, thereby contributing to the reduction of the associated dB levels at various frequencies. Analysis of the C-shaped solder pad design versus the spoked solder pad design shows that the total surface area of the C-shaped solder pad design that comes into contact with the computer chassis is significantly different than that of the spoked solder pad design.
While the solder pad designs of the prior art do provide workable solutions to the problem of solder occlusion, the C-shaped solder pad design has variations-in-thickness problems resulting from variations in the orientation of the solder pad. Moreover, the spoked solder pad design has insufficient contact area problems which can result in an improper grounding connection.
In other words, the C-shaped solder pad design provides sufficient contact area between the solder pad on the lower surface of the printed circuit board and the computer chassis to provide for proper grounding with the chassis. However, the varying orientation of the asymmetric C-shaped solder pad design as it is passed through the wave solder creates variations in the thickness of the solder on the solder pad.
On the other hand, while the spoked design does not have this orientation problem, it does have a drawback in that there is insufficient contact area between the solder pad on the lower surface of the printed circuit board and the computer chassis to provide for a proper grounding connection. Therefore, a solution is needed that addresses all of the problems which currently exist in the prior art.